Old values not appearing in text field when called - php

Question

I'm trying to call the old values to be edited. What part am I wrong at?
<?php
if (isset($_GET['edit'])) {
$id = $_GET['edit'];
$update = true;
$record = mysqli_query($db, "SELECT * FROM bookinfo WHERE BookNo='$BookNo'");
if (mysqli_num_rows($record) == 1 ) {
$n = mysqli_fetch_array($record);
$BookNo = $n['BookNo'];
$ISBN = $n['ISBN'];
$title = $n['title'];
$author = $n['author'];
$publisher = $n['publisher'];
$status = $n['status'];
$cost = $n['cost'];
}
}
?>
<a href="viewBook.php?edit=<?php echo $row['BookNo']; ?>" class="edit_btn" >Edit</a>
</td>
<?php
if (isset($_GET['edit'])) { ?>
<form method="post" action = "viewBook.php">
<input type="hidden" name="BookNo" value="<?php echo $BookNo; ?>">
<input type="text" name="ISBN" value="<?php echo $ISBN; ?>">
<input type="text" name="title" value="<?php echo $title; ?>">
<input type="text" name="author" value="<?php echo $author; ?>">
<input type="text" name="publisher" value="<?php echo $publisher; ?>">
<input type="text" name="status" value="<?php echo $status; ?>">
<input type="text" name="cost" value="<?php echo $cost; ?>">
<?php if ($update == true): ?>
<button class="btn" type="submit" name="update" style="background: #556B2F;" >update</button>
<?php else: ?>
<button class="btn" type="submit" name="save" >Save</button>
<?php endif ?>
<?php } ?>
</form>
So far, what it does is, when the user clicks the edit button, it just shows 6 text fields. I thought by doing what I did, it was supposed to show the details already filled in the textbox.

Answer 1

When you do
$record = mysqli_query($db, "SELECT * FROM bookinfo WHERE BookNo='$BookNo'");
$BookNo is not defined.
maybe you wanted to do something like this:
$id = $_GET['edit'];
$update = true;
$record = mysqli_query($db, "SELECT * FROM bookinfo WHERE BookNo='$id'");

Answer 2

<form method="post" action = "viewBook.php">
your form method is "post" but you are checking $_GET You must check $_POST
if (isset($_GET['edit']))

Answer 3

you are passing value in $id And using $BookNo which not define.
only 6 input field will be show because first one is using hidden property.
<input type="hidden" name="BookNo" value="<?php echo $BookNo; ?>">
when you click on submit button data will be receive by $_POST